Abstract
The interfacial premelting in ice/clay nano composites was studied by high energy X-ray diffraction. Below the melting point of bulk water, the formation of liquid water was observed for the ice/vermiculite and ice/kaolin systems. The liquid fraction is gradually increasing with temperature. For both minerals, similar effective premelting layer thicknesses of 2-3 nm are reached 3 K below the bulk melting point. For the quantitative description of the molten water fraction in wet clay minerals we developed a continuum model for short range interactions and arbitrary pore size distributions. This model quantitatively describes the experimental data over the entire temperature range. Model parameters were obtained by fitting using a maximum entropy (MaxEnt) approach. Pronounced differences in the deviation from Antonow's rule relating interfacial free energy between ice, water, and clay are observed for the charged vermiculite and uncharged kaolin minerals. The resultant parameters are discussed in terms of their ice nucleation efficiency. Using well defined and characterized ice/clay nano composite samples, this work bridges the gap between studies on single crystalline ice/solid model interfaces and naturally occurring soils and permafrost.
Highlights
As of today, different physical effects contributing to the interfacial premelting phenomenon have been identified.[5,7] In ice composites, the most relevant ones are the intrinsic interfacial melting, impurities, confinement, and geometry effects (Fig. 1)
In the low temperature region, liquid water is still detected below À20 1C. For both kaolin samples we find consistently larger d% values compared to vermiculite at the same temperature
At Tm À T = 1.0 K the liquid fractions f (Tm – T) lie in between 0.2 and 0.4. This corresponds to premelting layer thicknesses of 3.5 nm for vermiculite and 7.5 nm for kaolin samples
Summary
Different physical effects contributing to the interfacial premelting phenomenon have been identified.[5,7] In ice composites, the most relevant ones are the intrinsic interfacial melting, impurities, confinement, and geometry effects (Fig. 1). Planar, single crystalline ice surfaces and ice/solid interfaces were investigated by glancing angle X-ray scattering,[15] X-ray reflectivity,[16,17,18] X-ray photoemission spectroscopy,[19] sum frequency generation spectroscopy,[20] and molecular dynamics simulations.[21,22] Premelting in nanosized ice/solid composites was studied by thermodynamic measurements,[23,24] X-ray[25] and neutron diffraction,[26] quasi elastic neutron scattering,[27] NMR,[28] and time-domain reflectometry.[29] the liquid layer thicknesses obtained by the different experimental techniques and computer simulations differ by almost two orders of magnitude.[5,6] In particular, for the class of ice composites, most studies focused on the premelting in naturally occurring samples from permafrost regions with complex composition and morphology.[30,31] In general, this precludes a quantitative discussion of the intrinsic premelting mechanisms in terms of physical models. From comparison between the results obtained for charged vermiculite and uncharged kaolin clays we extract information on the relevant molecular interactions governing the intrinsic interfacial melting mechanism
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